This invention relates to oxide-based materials that have one application as phosphors. More particularly, the phosphors are aluminates or borates doped with Pr3+ and exhibit quantum splitting when irradiated with vacuum ultraviolet (xe2x80x9cVUVxe2x80x9d) radiation. This invention also relates to a method of making such quantum-splitting phosphors.
The conversion of a single ultraviolet (xe2x80x9cUVxe2x80x9d) photon into two visible photons with the result that the quantum efficiency of luminescence exceeds unity is termed quantum splitting. Quantum splitting materials are very desirable for use as phosphors for lighting applications, such as fluorescent lamps. A suitable quantum splitting phosphor can, in principle, produce a significantly brighter fluorescent light source due to higher overall luminous output because it can convert to visible light the part of UV radiation that is not absorbed efficiently by traditional phosphors currently used in commercial fluorescent lamps. Quantum splitting has been demonstrated previously in fluoride- and oxide-based materials. A material comprising 0.1% Pr3+ in a matrix of YF3 has been shown to generate more than one visible photon for every absorbed UV photon when excited with radiation having a wavelength of 185 nm. The measured quantum efficiency of this material was 140%, and thus greatly exceeded unity. However, fluoride-based compounds do not have sufficient stability to permit their use as phosphors in fluorescent lamps because they are known to react with mercury vapor that is used in such lamps to provide the UV radiation and form materials that do not exhibit quantum splitting. In addition, producing fluoride-based materials presents a great practical challenge because it involves the use of large quantities of highly reactive and toxic fluorine-based materials.
The applicants recently disclosed oxide-based quantum splitting materials. U.S. Pat. No. 5,552,082 discloses a lanthanum magnesium borate activated with Pr3+ ion. U.S. Pat. No. 5,571,4151 discloses a strontium magnesium aluminate activated with Pr3+ ion and charge compensated with Mg2+ ion. Emission spectra of these materials exhibit a large peak at about 405 nm which is characteristic of quantum splitting. However, these materials still exhibit a considerable emission in the UV wavelength range of less than 350 nm. This part of the emission reduces the overall visible light output that otherwise can be higher. Therefore, it is desirable to provide oxide-based quantum-splitting phosphors that have higher quantum efficiency in the visible range than the prior-art quantum splitting materials. It is also desirable to provide more energy-efficient light sources using quantum-splitting phosphors having higher quantum efficiency. It is further desirable to provide method for making materials having high quantum splitting capability.
The present invention provides oxide-based phosphors doped with Pr3+ ion, which phosphors exhibit quantum splitting when irradiated with VUV radiation. VUV radiation as used herein is radiation having wavelength shorter than about 215 nm. The oxide phosphors of the present invention are oxides of aluminum or boron having positive counterions selected from Group IIA of the Periodic Table. The phosphors of the present invention may be used in mercury vapor discharge lamps to provide energy-efficient light sources.
In one aspect of the present invention, the oxide-based phosphors are strontium or strontium calcium aluminates having the magnetoplumbite crystal structure. The aluminates are doped with Pr3+ ion. Furthermore, it is advantageous to substitute some of the aluminum ions with magnesium ions for the purpose of charge compensation when Pr3+ is substituted on the Sr2+ sites. Such oxide-based phosphors of the present invention have a composition represented by Sr1xe2x88x921.5yPryAl12O19, Sr1xe2x88x92xxe2x88x921.5yCaxPryAl12O19, or Sr1xe2x88x92xxe2x88x92zCaxMgzAl12xe2x88x92zPrzO19 where 0 less than x less than 1, y is in the range from about 0.005 to about 0.5, z is in the range from about 0.005 to about 0.5, x+1.5yxe2x89xa61, and x+z less than 1.
In another aspect of the present invention, the oxide-based phosphors are calcium or calcium magnesium aluminates activated with Pr3+ ion having a composition represented by Ca1xe2x88x92zPrzAl12O19, Ca1xe2x88x92zPrzMgAl11.33O19, or Ca1xe2x88x92zPrzMgAl14O23 where z is in the range from about 0.005 to about 0.5. In all of these host lattices, the Pr3+ ion can be charge compensated by the Mg+ ion or by lattice vacancies.
In another aspect of the present invention, the oxide-based phosphors are strontium borate activated with Pr3xe2x88x92 having a composition represented by Sr1+zPrzB4O7 where z is in the range from about 0.005 to about 0.5.
The present invention also provides a method of making improved quantum-splitting aluminate or borate phosphors. The method comprises the steps of selecting a desired final composition of the phosphor; mixing together materials from the following two groups: (1) at least one oxygen-containing compound of praseodymium and (2) materials selected from the group consisting of oxygen-containing compounds of strontium, calcium, magnesium, aluminum, and boron so to achieve the desired final composition; forming a substantially homogeneous mixture of the selected compounds; and firing the substantially homogeneous mixture in a non-oxidizing atmosphere at a temperature and for a time sufficient to result in the desired composition and to maintain the praseodymium ion in the 3+ valence state.
In another aspect of the present invention, the method further comprises adding at least one compound selected from the group consisting of fluoride salts of aluminum, calcium, and strontium in a quantity sufficient to act as a flux prior to the step of forming the substantially homogeneous mixture. When the oxide-based phosphor is a borate, a quantity of boric acid may be advantageously used, either in place of or in combination with the fluoride salts, as the flux.
Other benefits of this invention may become evident by a perusal of the description and appended claims together with the attached drawings.